Electrophysiology and enkephalin immunoreactivity of identified myenteric plexus neurones of guinea-pig small intestine.

Intracellular injection of the fluorescent dye, Lucifer Yellow CH, revealed the shapes of neurones in the myenteric plexus of the guinea-pig ileum, and these shapes were correlated with the electrophysiological properties and enkephalin immunoreactivity of the neurones. A total of eighty-three neurones were filled using electrodes containing a 5% solution of the dye. Forty-six cells had many short processes and a single long process (Dogiel type 1) and twenty-four cells had essentially smooth somas and one to eight long processes (Dogiel type II). Thirteen cells could not be put into either group. Enkephalin-like immunoreactivity was detected in twenty-two of the forty-six Dogiel type I cells. Eighteen of these had club-like short processes. No other cells of the eighty-three showed enkephalin-like immunoreactivity. Electrodes filled with a 0.5% solution of Lucifer Yellow in 0.5 M-KCl were used to record from and simultaneously to inject dye into 240 neurones. Eighty-six nerve cells had a slow after-hyperpolarization following the action potential (AH cells) and forty-six nerve cells had no after-hyperpolarization but exhibited a fast excitatory synaptic potential (S cells). The other cells could not be unequivocally identified by their observed electrophysiological characteristics. Almost all S cells (forty-two of forty-six) were Dogiel type I, while eighty-two of the eighty-six AH cells were Dogiel type II. Fifty S cells (eight located geometrically, forty-two by dye injection) and ninety-one AH cells (twenty-six located geometrically, sixty-five by dye injection) were examined for enkephalin immunoreactivity. Fifteen of the S cells were reactive, whereas all of the AH cells were unreactive. It appears that prolonged impalements reduce immunoreactivity so that the proportion of reactive neurones in this series is an underestimate of the true proportion of S cells with enkephalin-like immunoreactivity. The results suggest that a substantial proportion of the S cells in myenteric ganglia contain enkephalin immunoreactivity while none of the AH cells do. The enkephalin neurones have a distinctive shape and are all Dogiel type I cells. AH cells are nearly always Dogiel type II.